High density package substrate and method for fabricating the same
The invention provides a high density package substrate and a method for fabricating the same. A double-sided copper clad laminate containing an upper copper foil and a lower copper foil is provided. A bottom pad is disposed on the lower copper foil, aligned to a predetermined position of a through hole. The through hole is formed by laser drilling through the upper copper foil and the substrate, but not through the bottom pad. A seed layer is formed conformally lining the through hole, and a metal layer is formed on the seed layer by plating to form a plated through hole (PTH).
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This Application claims priority of Taiwan Patent Application No. 097118079, filed on May 16, 2008, the entirety of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a high density package substrate and method for fabricating the same, and in particular relates to a method for forming a high density package substrate by laser drilling.
2. Description of the Related Art
Along with technological development, electrical devices are becoming lighter, thinner, shorter and smaller. As such, printed circuit boards in electrical devices are also being required to become lighter, thinner, shorter and smaller. Thus, high density interconnection (HDI) technology, allowing for high density interconnection, fine spacers and miniaturization, is being used in printed circuit boards to provide increased functions in a limited space and further decrease fabrication process costs. For HDI technology, via-on-pad, with a plated through hole formed on a pad, is utilized. Additionally, because a conventional mechanical drill destroys copper foils of a double-sided copper clad laminate, laser drills are used to more accurately control the location and size of holes for the via-on-pad.
There are three main methods for forming the plated through hole of the double-sided copper clad laminate. The first method is a full-subtractive process, the second method is a semi-subtractive process and the third method is a modified semi-additive process.
The thickness of the double-sided copper clad laminate copper foil used in the full-subtractive process is generally about 12 μm (or 18 μm). When the through hole is formed by laser drilling, the thickness of the lower copper foil is at least larger than 12 μm, so that a punch through problem caused by a laser drill is prevented. However, on the other hand, the thick copper foil increases etching time. Therefore, increasing etching time worsens an undercut phenomenon of the printed circuit board and negatively affects the forming of fine lines.
The thickness of the double-sided copper clad laminate copper foil used in the semi-subtractive process and the modified semi-additive process is generally only about 4 μm. The advantage of both processes is that the thinner copper foil improves the undercut phenomenon of the printed circuit board and improves forming of fine lines. However, the ultra-thin copper foil causes other problems, such as complicating the laser drilling process, increasing costs and increased probability of punch through.
Accordingly, there is a need to develop a process, which not only prevents the punch through caused by laser drilling, but also allows the double-sided copper clad laminate to have fine line capability.
BRIEF SUMMARY OF THE INVENTIONAn object of the invention is to provide a high density package substrate and method for fabricating the same, which can prevent punch through caused by laser drilling and allow the substrate to have fine line capability.
The present invention provides a method for forming a high density package substrate, comprising: providing a double-sided copper clad laminate having an upper copper foil and a lower copper foil; forming a bottom pad on the lower copper foil, wherein the bottom pad is aligned to a predetermined position of a through hole; forming the through hole by laser drilling, wherein the through hole passes through the upper copper foil and the substrate, but not through the bottom pad; forming a seed layer conformally on the through hole; and forming a plated through hole by plating a metal on the seed layer.
The present invention also provides a high density package substrate, comprising: a substrate; an upper copper foil formed on the upper side of the substrate; a seed layer conformally formed on the through hole; a metal disposed on the seed layer and filled the through hole; a lower copper foil formed on a lower surface of the substrate; and a bottom pad directly formed on the lower copper foil and aligned to the through hole.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
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According to the above description, the method for fabricating the high density package substrate of the invention has two major advantages:
(1) Because the bottom pad is formed on the lower copper foil in advance, the punch through problem caused by laser drilling is prevented. The probability of the punch through caused by laser drilling and the cost of the laser process of the invention are decreased as compared with the conventional semi-subtractive method.
(2) Since the upper or lower copper foil of the substrate is an ultra-thin copper foil, the etching time of the printed circuit board is decreased, the undercut phenomenon is insignificant and the printed circuit board has fine line capability. The fine line capability of the invention is better than the conventional semi-subtractive method.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Claims
1. A high density package substrate, comprising:
- a substrate;
- an upper copper foil formed on the upper side of the substrate;
- a lower copper foil formed on a lower surface of the substrate;
- a through hole through the upper copper foil and the substrate;
- a seed layer conformally formed on the through hole, the upper foil and the lower foil;
- a metal disposed on the seed layer and filled the through hole; and
- a bottom pad directly formed on the lower copper foil and aligned to the through hole, wherein the bottom pad is covered by the seed layer.
2. The high density package substrate as claimed in claim 1, wherein the substrate comprises a core material of paper phenolic resin, composite epoxy, polyimide resin or glass fiber.
3. The high density package substrate as claimed in claim 1, wherein each of the upper copper foil and the lower copper foil has a thickness of smaller than 4 mm.
4. The high density package substrate as claimed in claim 1, wherein the through hole is formed by laser drilling.
5. The high density package substrate as claimed in claim 4, wherein the laser drill uses a CO2 laser.
6. The high density package substrate as claimed in claim 1, wherein the seed layer comprises copper, tantalum or combinations thereof.
7. The high density package substrate as claimed in claim 1, wherein the bottom pad comprises copper, aluminum, nickel, gold or combinations thereof.
8. The high density package substrate as claimed in claim 1, wherein the metal comprises copper, aluminum, nickel, gold or combinations thereof.
9. The high density package substrate as claimed in claim 1, wherein the bottom pad has an area larger than the size of the through hole.
10. The high density package substrate as claimed in claim 1, wherein the bottom pad has a thickness of larger than 12 μm.
11. The high density package substrate as claimed in claim 1, wherein no seed layer is provided between the bottom pad and the lower copper foil.
20040136152 | July 15, 2004 | Mitsuhashi et al. |
20080314632 | December 25, 2008 | Wu et al. |
Type: Grant
Filed: Jul 17, 2008
Date of Patent: Nov 15, 2011
Patent Publication Number: 20090283315
Assignee: Nan Ya PCB Corp. (Taoyuan County)
Inventors: Meng-Han Lee (Taoyuan County), Wei-Wen Lan (Taoyuan County), Ching-Ming Chuang (Taoyuan County), Shi-Shyan James Shang (Taoyuan County)
Primary Examiner: Dameon Levi
Assistant Examiner: Hoa C Nguyen
Application Number: 12/175,094
International Classification: H05K 1/11 (20060101);